Methods and systems for assisted direct start control

ABSTRACT

Methods and systems are provided for controlling a vehicle system including a selectively shut-down engine, a torque converter and a torque converter lock-up clutch. One example method comprises, during an idle-stop engine shut-down, restricting flow of transmission fluid out of the torque converter, and adjusting engagement of the torque converter lock-up clutch to adjust a drag torque on the engine to stop the engine.

FIELD

The present application relates to methods and systems for controllingan engine shut-down.

BACKGROUND AND SUMMARY

Vehicles have been developed to perform an idle-stop when idle-stopconditions are met and automatically restart the engine when restartconditions are met. Such idle-stop systems enable fuel savings,reduction in exhaust emissions, reduction in noise, and the like. Assuch, a number of methods may be used to control the transmission toimprove idle-stops and subsequent restarts, when restart conditions aremet.

One such example is illustrated by Suzuki et al. in U.S. Pat. No.6,556,910 B2. Therein, a plurality of transmission forward clutches arecontrolled by a hydraulic servo to shift the clutches between engagedand disengaged states when adjusting between idle-stop and restartconditions. Specifically, when an idle-stop condition is satisfied, thetransmission is maintained in gear and a hydraulic pressure of thehydraulic servo is also maintained at a predetermined pressure.

However, the inventors have recognized several potential issues withsuch a method. As one example, during idle-stop conditions, the timerequired to stop the engine, for example the time required to drop theengine speed from 700 RPM to zero, may be longer than desired. As such,if the time needed for engine shut-down is too long, a vehicle operatormay choose to restart and/or launch the vehicle before the engine speedhas dropped to zero.

Thus in one example, some of the above issues may be addressed by amethod of controlling a vehicle system including an engine that may beselectively shut down, the system further including a torque converterand a torque converter lock-up clutch. One example embodiment comprises,during an idle-stop engine shut-down, restricting flow of transmissionfluid out of the torque converter, and adjusting engagement of thetorque converter lock-up clutch to adjust a drag torque on the engine tostop the engine.

In one example, a flow restriction valve may be included in thehydraulic circuit of a torque converter to thereby restrict (forexample, fully restrict or partially restrict) the flow of transmissionfluid out of the converter. As such, restricting flow out of the torqueconverter may include restricting a flow of transmission fluid into asystem cooler and/or lube. The position of the flow restriction valvemay be varied based on the nature of the torque converter. For example,when the torque converter is a two-pass torque converter, the flowrestriction valve may be positioned in a converter release circuit.Alternatively, when the torque converter is a three-pass orclosed-piston type torque converter, the flow restriction valve may bepositioned in a clutch out circuit. During an idle shut-down operation,that is, when the engine is shut-down responsive to idle-stop conditionsand without receiving a shut-down request from the vehicle operator, anengine controller may close the flow restriction valve to enable anincrease in the torque capacity of the torque converter lock-up clutch.As such, during an engine idle-stop shut-down, the drop in engine speedleads to a corresponding drop in output from an engine-driven mechanicalpump. The consequent drop in hydraulic pressure may reduce the capacityof transmission clutches, such as the torque converter lock-up clutch.Herein, by closing the flow restriction valve during the engineshut-down, at least some clutch capacity may be restored (for example,less than full capacity may be restored), by restoring at least somehydraulic pressure.

To expedite engine shut-down, the controller may further command thetorque converter to be locked up, by adjusting an engagement of thetorque converter lock-up clutch, to thereby apply a drag torque on theengine to stop the engine. A degree of engagement of the torqueconverter lock-up clutch may be adjusted responsive to operatingconditions, such as an engine speed and/or a desired stopping positionof the engine. In one example, when the engine speed is above a desiredengine speed, the engagement of the torque converter lock-up clutch maybe increased to increase the drag torque applied. In another example,when the engine speed is below a desired engine speed, the engagement ofthe torque converter lock-up clutch may be decreased to decrease thedrag torque applied on the engine. In one example, the increasedengagement of the torque converter lock-up clutch and the closedposition of the flow restriction valve may be maintained until shut-downhas been completed. Then, after completing the shut-down, the flowrestriction valve may be opened to un-restrict flow of transmissionfluid out of the torque converter, the engagement of the torqueconverter lock-up clutch may be reduced (for example, the lock-up clutchmay be disengaged), and the torque converter may be unlocked. During asubsequent engine restart, the engine may be cranked with the torqueconverter lock-up clutch in the reduced engagement condition (ordisengaged condition) and the flow restriction valve open. Then, as theengine speed rises, with the flow restriction valve open, the engagementof one or more transmission clutches may be modulated (for example, theengagement of a forward clutch may be increased). The flow restrictionvalve may also enable improved pressure control during conditions whenthe transmission fluid has lower flow rates through the torqueconverter.

In this way, a torque converter lock-up clutch may be advantageouslyused to apply a drag torque and expedite engine shutdown even duringconditions of reduced pump output to the clutches. Specifically, byusing a flow restriction valve to reduce flow of transmission fluid outof the torque converter, hydraulic pressure and torque-converter clutchcapacity may be maintained even during conditions of reduced pumpoutput. Furthermore, a duration of torque converter lock-up duringengine shut-down conditions may be increased (for example, by increasinga duration of torque converter lock-up clutch engagement). By adjustinga degree and/or duration of engagement of a torque converter lock-upclutch, a drag torque may be applied to counteract a rotation of theengine by the ground, through the wheels and/or powertrain, therebyproviding a faster engine shut-down. In addition to enabling a fasterengine shut-down, crankshaft oscillations due to cylinder air-springeffects after the engine speed had reached zero, may be significantlydampened. In this way, repeated stop/start events may be bettersupported.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example vehicle system layout, including details of avehicle drive-train.

FIGS. 2A-C show example embodiments of a torque converter hydrauliccircuit.

FIG. 3 shows a high level flow chart for a torque converter lock-upoperation.

FIG. 4 shows a high level flow chart for executing an idle-stopoperation by applying a torque converter lock-up clutch-based dragtorque according to the present disclosure.

FIG. 5 shows a high level flow chart for executing a restart operationaccording to the present disclosure.

FIG. 6 shows a map with a plurality of graphs explaining example engineshut-down and restart procedures, according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for expeditingengine shut-down, when idle-stop conditions are met, by applying atorque converter clutch based drag torque on the engine. Specifically, aflow restriction valve may be included in the hydraulic circuit of atorque converter (FIG. 2) to adjust (for example, restrict orun-restrict) a flow of transmission fluid there-through. By adjustingflow through the torque converter, an actuation and/or degree ofengagement of a torque converter lock-up clutch (TCC) may be adjusted.In this way, the torque converter lock-up clutch may be used as a dragtorque actuator to expedite and/or control an engine shut-down process(FIG. 4). Similarly, during an engine restart process (FIG. 5), aTCC-based drag torque may be applied to rapidly reduce the engine speedto a threshold value, from where an engine starter may be rapidlyactuated. By restricting flow only during idle-stop conditions (FIG. 3),the flow and pressure characteristics of transmission fluid in thetorque converter may be modulated to thereby adjust an engagement stateof the torque converter lock-up clutch. The concepts and routinesintroduced herein are further clarified with example engine shut-downand restart scenarios in FIG. 6. In this way, by applying a TCC-baseddrag torque during engine idle-stop conditions, the time required forshutting down an engine may be significantly reduced, thereby providingfuel efficiency benefits and better enabling frequent enginestart/stops.

FIG. 1 is a block diagram of a vehicle drive-train 20. Drive-train 20may be powered by engine 22. In one example, engine 22 may be a gasolineengine. In alternate embodiments, other engine configurations may beemployed, for example a diesel engine. Engine 22 may generate or adjusttorque via torque actuator 26, such as a fuel injector, throttle, etc.

Engine 22 may include an auxiliary starter system 24 to support enginerestart at near zero engine speed, for example at 50 RPM. In oneexample, the auxiliary starter system 24 may be used to restart theengine if a driver requests a vehicle launch while the engine is beingspun down in response to prior fulfillment of idle-stop conditions.Auxiliary starter systems, however, may add significant cost andcomplexity to the engine system. Thus, in one example, by using a dragtorque to expedite engine shut down, the requirement for such auxiliarystarter systems may be reduced. Consequently, the cost and complexityincurred by such starter systems in vehicle drive-train 20 may bereduced.

An engine output torque may be transmitted to torque converter 28 todrive an automatic transmission 30 by engaging one or more hydraulicallyactuated transmission components, or clutches, including forward clutch32. As such, a plurality of such clutches may be engaged, as needed.Herein, torque converter 28 may also be referred to as a component ofthe automatic transmission 30. As further elaborated with reference toFIGS. 2A-C, torque converter 28 may be a two-pass, three-pass, orclosed-piston type torque converter. The lock-up state of torqueconverter 28 may in turn be controlled by torque converter lock-upclutch 34 to thereby vary the torque output from the torque converter.As such, when torque converter lock-up clutch 34 is fully disengaged,torque converter 28 is unlocked. In this state, torque converter 28transmits torque to automatic transmission 30 via fluid transfer betweenthe torque converter turbine and torque converter impeller, therebyenabling torque multiplication. In contrast, when torque converterlock-up clutch 34 is fully engaged, torque converter 28 is locked-up. Inthis state, the engine output torque is directly transferred via thetorque converter clutch to an input shaft (not shown) of transmission30. Alternatively, torque converter lock-up clutch 34 may be partiallyengaged, thereby enabling the amount of torque relayed to thetransmission to be modulated. A controller 40 may be configured toadjust an amount of torque transmitted by the torque converter byadjusting the actuation (or engagement state) of torque converterlock-up clutch 34 in response to various engine operating conditions,for example, an engine speed. In one example, the engagement state ofthe clutch (or degree of engagement of the clutch) may be varied byvarying a clutch pressure and a direction of transmission fluid flowthrough the torque converter lock-up clutch.

Torque output from automatic transmission 30 may in turn be relayed towheels 36 to propel the vehicle. Specifically, automatic transmission 30may adjust an input driving torque at the input shaft (not shown)responsive to a vehicle traveling condition before transmitting anoutput driving torque to the wheels 36. Further, wheels 36 may be lockedby engaging wheel brakes 38. In one example, wheel brakes 38 may beengaged in response to the driver pressing his foot on a brake pedal(not shown). In the same way, wheels 36 may be unlocked by disengagingwheel brakes 38 in response to the driver releasing his foot from thebrake pedal.

An engine-driven mechanical oil pump 42 may be in fluid communicationwith the automatic transmission 30 to provide hydraulic pressure toengage various clutches, such as a forward clutch 32 and/or torqueconverter lock-up clutch 34. Mechanical oil pump 42 may be driven by therotation of engine 22 or transmission input shaft, for example. Thus,the hydraulic pressure generated by mechanical oil pump 42 may increaseas an engine speed increases, and may decrease as an engine speeddecreases.

Controller 40 may be configured to receive inputs from engine 22 andaccordingly control a torque output of the engine and/or operation ofthe torque converter, transmission, and/or brakes. As one example, atorque output may be controlled by adjusting a combination of sparktiming, fuel pulse width, fuel pulse timing, and/or air charge, bycontrolling throttle opening and/or valve timing, valve lift and boostfor turbocharged or supercharged engines. In the case of a dieselengine, controller 40 may control the engine torque output bycontrolling a combination of fuel pulse width, fuel pulse timing, andair charge. In all cases, engine control may be performed on acylinder-by-cylinder basis to control the engine torque output.

When idle-stop conditions are satisfied, controller 40 may controloperation of the transmission components to control stopping of theengine. In one example, to decrease the duration of the engine spindown, a controllable drag torque may be applied to the engine via thetransmission 30 and the torque converter 28. Specifically, the torqueconverter 28 may be used to transmit a drag torque generated by thestopped vehicle wheels through the gears of the transmission via torqueconverter lock-up clutch 34. In other words, an in-gear transmission maybe used to apply and adjust an external friction torque (or drag torque)on the engine. In one example, when the torque converter lock-up clutchis fully engaged, and the wheels are held fixed to the ground viafriction and/or the wheel brakes, a large drag torque can be applied tothe engine (assuming the wheels do not break free from the ground). Alarger drag torque may be applied if one or more alternate transmissionclutches are additionally engaged, such as, for example, thetransmission forward clutch. Similarly, the drag torque may be reducedby increasing the slip of at least the torque converter lock-up clutch.

The TCC-based drag torque applied can also be modulated by adjusting adegree of engagement of the torque converter lock-up clutch 34. Forexample, a larger drag torque can be generated by increasing engagementof the torque converter lock-up clutch while a smaller drag torque canbe generated by decreasing engagement of the torque converter lock-upclutch. The degree of engagement of torque converter lock-up clutch 34may be adjusted responsive to engine operating conditions, such asengine speed. For example, a controller may increase the degree ofengagement of torque converter lock-up clutch 34 to increase the applieddrag torque in response to the engine speed being above a desired enginespeed, and decrease the degree of engagement of the clutch to decreasethe applied drag torque in response to engine speed being below thedesired engine speed. In one example, the desired engine speed may be athreshold speed below which a starter can be actuated to restart theengine (for example, in response to a sudden vehicle restart and/orre-launch request from the driver).

As further elaborated with reference to FIG. 3, the engagement of thetorque converter lock-up clutch may be controlled by selectivelyadjusting a direction of flow and a hydraulic pressure of transmissionfluid through the torque converter. As such, during an engine shut down,the drop in engine speed leads to a corresponding drop in pump output.This, in turn, leads to a drop in hydraulic pressure through the torqueconverter lock-up clutch, and a drop in clutch capacity. By closing aflow restriction valve (FIGS. 2A-C) included in the hydraulic circuit ofthe torque converter during the engine shut-down, at least some torqueconverter lock-up clutch capacity may be restored (for example, lessthan full capacity may be restored). This improvement in flow andpressure of transmission fluid through the torque converter lock-upclutch enables the TCC-based drag torque to be applied and adjustedduring an engine shut-down even during reduced pump output.

Now turning to FIGS. 2A-C, example embodiments of a torque converterhydraulic circuit are described. In each embodiment, the direction offlow of transmission fluid through the hydraulic circuit determines astate of engagement of the torque converter clutch, and thereby, alock-up state of the torque converter.

FIG. 2A depicts a first torque converter hydraulic circuit 100 includinga two-pass torque converter 128 coupled to a torque converter lock-upclutch 134. The two-pass torque converter may include two hydrauliclines providing flow of transmission fluid in and out of torqueconverter 128 via torque converter lock-up clutch 134. Specifically,converter apply circuit 102 is configured to supply transmission fluidfrom a pump into torque converter lock-up clutch 134 while converterrelease circuit 104 is configured to exhaust transmission fluid fromtorque converter lock-up clutch 134 into a cooler and/or lube. As such,the direction of flow determines the engagement state of torqueconverter lock-up clutch 134. Specifically, flow of transmission fluidinto converter apply circuit 102 enables torque converter lock-up clutch134 to be engaged while flow of transmission fluid into converterrelease circuit 104 enables torque converter lock-up clutch 134 to bedisengaged (or released). As previously elaborated, when torqueconverter lock-up clutch 134 is engaged, torque converter 128 may belocked-up. In contrast, when torque converter lock-up clutch 134 isdisengaged, torque converter 128 may be unlocked.

Flow through converter apply circuit 102 and converter release circuit104 may be regulated, at least in part, by clutch apply regulatory valve112. In one example, clutch apply regulatory valve 112 may be a flowregulator spool valve controlled by a dedicated pressure controlsolenoid or variable force solenoid. In response to a first torqueconverter lock-up clutch solenoid command (for example, energization orde-energization), clutch apply regulatory valve 112 may directtransmission fluid into converter apply circuit 102 to engage torqueconverter lock-up clutch 134. The solenoid command may, for example,position the spool valve by applying a controlled pressure to the spoolthat is reacted against by the return spring and flow forces. Similarly,in response to a second, alternate torque converter lock-up clutchsolenoid command, clutch apply regulatory valve 112 may reverse the flowdirection, directing transmission fluid into converter release circuit104 to disengage torque converter lock-up clutch 134. Further, byadjusting a degree of opening of clutch apply regulatory valve 112, flowand hydraulic pressure in converter apply circuit 102 may be modulatedto provide partial engagement of torque converter lock-up clutch 134. Todisengage the clutch, that is, in the release flow direction, it may notbe necessary to modulate the release pressure. Specifically, it may onlybe necessary to apply sufficient pressure to release the clutch.

Flow restriction valve 120 may also be included in hydraulic circuit100. Specifically, flow restriction valve 120 may be located inconverter release circuit 104, and positioned upstream of a systemcooler and/or lube, to restrict or un-restrict exhaust flow oftransmission fluid into the cooler. In one example, during enginerunning conditions, flow restriction valve 120 may remain open, therebyun-restricting flow out of the torque converter lock-up clutch, andclutch apply regulatory valve 112 may modulate flow and pressure oftransmission fluid into the torque converter lock-up clutch. In anotherexample, during engine shut-down conditions (that is, reduced pumpoutput conditions), flow restriction valve 120 may be closed (forexample, at least partially closed), thereby restricting flow out oftorque converter lock-up clutch 134 and restricting flow into thecooler, to thereby maintain flow and pressure of transmission fluidthrough the torque converter lock-up clutch. In one example, restrictingflow may include fully closing flow restriction valve 120. Herein, theflow restriction valve 120 may be operated as an on/off flow controlvalve. In another example, restricting flow may include partiallyclosing flow restriction valve 120. Subsequently, the flow and pressurein converter apply circuit 102 and a degree of engagement of torqueconverter lock-up clutch 134 may be modulated by adjusting clutch applyregulatory valve 112.

FIG. 2B depicts a second torque converter hydraulic circuit 200including a three-pass torque converter 228 coupled to a torqueconverter lock-up clutch 234. The three-pass torque converter includesthree hydraulic lines providing transmission fluid in and out of torqueconverter 228 via torque converter lock-up clutch 234. Specifically,converter apply circuit 202 and converter release circuit 204 supplytransmission fluid from a pump into the torque converter lock-up clutch234 while converter open circuit 206 exhausts transmission fluid into acooler and/or lube. Flow through converter apply circuit 202 and/orconverter release circuit 204 may be regulated by clutch applyregulatory valve 212. Alternatively, flow through converter releasecircuit 204 may be regulated by an optional clutch release regulatoryvalve 214.

The direction of fluid flow determines an engagement state of the torqueconverter lock-up clutch 234. Specifically, flow of transmission fluidfrom converter apply circuit 202 to converter open circuit 206 enablesengagement of torque converter lock-up clutch 234 (and lock-up of torqueconverter 228) while flow of transmission fluid from converter releasecircuit 204 to converter open circuit 206 enables disengagement oftorque converter lock-up clutch 234 (and unlocking of torque converter228). A switching valve (not shown) included in converter open circuit206 may enable the above mentioned switch in flow through converter opencircuit 206. Further, a degree of engagement may be varied by adjustingclutch apply regulatory valve 212.

Flow restriction valve 220 may be included in converter open circuit 206to restrict (for example, fully restrict or partially restrict) anexhaust flow of transmission fluid out of the torque converter lock-upclutch. Specifically, during engine shut-down conditions, flowrestriction valve 220 may be closed to restrict fluid flow out ofconverter open circuit 206, and to maintain flow and pressure oftransmission fluid through the torque converter lock-up clutch. Thus, inone example, flow restriction valve 220 may be operated as an on/offflow control valve. Subsequently, the flow and pressure in converterapply circuit 202 and thereby a degree of engagement of torque converterlock-up clutch 234 may be modulated by adjusting clutch apply regulatoryvalve 212.

FIG. 2C depicts a third torque converter hydraulic circuit 250 includinga closed-piston torque converter 278 coupled to a torque converterlock-up clutch 284. The closed-piston torque converter 278 may includethree hydraulic lines providing flowing of transmission fluid in and outof torque converter 278 via torque converter lock-up clutch 284.Specifically, converter apply circuit 252 and converter release circuit254 may supply transmission fluid from a pump into the torque converterlock-up clutch 284 while converter open circuit 256 may exhausttransmission fluid into a cooler and/or lube. Flow through converterapply circuit 252 and/or converter release circuit 254 may be regulatedby clutch apply regulatory valve 262. Alternatively, flow throughconverter release circuit 254 may be regulated by an optional clutchrelease regulatory valve 264.

Herein, in addition to the direction of flow, a pressure in converterapply circuit 252 and/or a pressure difference between converter applycircuit 252 and converter release circuit 254 may determine theengagement state of torque converter lock-up clutch 284. In one example,a pressure sensor 266 may estimate a pressure difference betweenconverter apply circuit 252 and converter release circuit 254. To engagetorque converter lock-up clutch 284 (and lock-up torque converter 278),transmission fluid may flow from converter apply circuit 252 toconverter open circuit. In comparison, to disengage torque converterlock-up clutch 284 (and unlock torque converter 278), transmission fluidmay flow from converter release circuit 254 to converter open circuit256. A switching valve (not shown) included in converter open circuit256 may enable the abovementioned switch in flow through converter opencircuit 256. Further, a degree of engagement of the clutch may be variedby adjusting clutch apply regulatory valve 262.

Flow restriction valve 270 may be included in converter open circuit 256to restrict (for example, fully restrict or partially restrict) exhaustflow of transmission fluid into the cooler. Specifically, during engineshut-down conditions, flow restriction valve 270 may be closed torestrict fluid flow out of converter open circuit 256, and to maintainflow and pressure of transmission fluid through the torque converterlock-up clutch 284. Subsequently, the flow and pressure in converterapply circuit 252 and a degree of engagement of torque converter lock-upclutch 284 may be modulated by adjusting clutch apply regulatory valve262.

While FIGS. 2A-C illustrate adjusting a flow rate and pressure of fluidin the torque converter using the restriction valve, it will beappreciated that in alternate embodiments, the inlet flow.

In each of the embodiments of FIG. 2A-C, during an engine shutdown,exhaust flow of transmission fluid from the torque converter into acooler and/or lube section of a vehicle may be restricted to therebymaintain pressure in the hydraulic circuit and maintain an engagementstate of the clutch. By using a restriction valve in the outlet of atorque converter clutch hydraulic circuit, the flow rate required togenerate a desired apply pressure (to thereby adjust an engagement stateof the clutch, and a torque output) may be reduced. In alternateembodiments, the flow rate may be adjusted using both the restrictionvalve in the outlet of the hydraulic circuit as well as a flow controlvalve (such as, the clutch apply regulatory valve) in the inlet of thehydraulic circuit. In this way, during engine spin down, the engagedtorque converter lock-up clutch may apply an increased drag torque onthe engine to expedite engine spin-down.

Now turning to FIG. 3, an example torque converter lock-up routine 300is described. The example routine may be used to lock-up a torqueconverter, by the engagement of a torque converter lock-up clutch, basedon the presence or absence of engine idle-stop conditions.

At 302, it may be confirmed whether a torque converter lock-up has beenrequested. In one example, torque converter lock-up may be requested toenable modulation of the engine torque output. If no lock-up has beenrequested, the routine may end. If a torque converter lock-up request isconfirmed, at 304, it may be determined whether the lock-up has beenrequested under engine shut-down conditions (for example, engineidle-stop conditions).

If engine shut-down conditions are not confirmed, that is, the torqueconverter lock-up has been requested during engine running conditions,at 306, exhaust flow out of the torque converter may be un-restricted,for example, by opening a flow restriction valve included in the torqueconverter hydraulic circuit. In one example, when the torque converteris a two-pass torque converter, the flow restriction valve may be openedto un-restrict flow out of a converter release circuit. In anotherexample, when the torque converter is a three-pass or closed-pistontorque converter, the flow restriction valve may be opened toun-restrict flow out of a converter open circuit.

At 308, the actuation of a torque converter lock-up clutch solenoid maybe adjusted, for example energized or de-energized based on theorientation of the solenoid, by appropriately adjusting the solenoid'sduty cycle. In one example, as depicted, the solenoid may be energized.In response to solenoid energization, at 310, a clutch apply regulatoryvalve (CARV) may adjust the hydraulic pressure and direction of fluidflow (as generated by the engine-driven mechanical pump) through thetorque converter lock-up clutch to enable an engagement of the clutchand a consequent lock-up of the torque converter. As previouslyelaborated with reference to FIGS. 2A-C, engagement of the torqueconverter lock-up clutch may include, at 312, the CARV adjusting a flowof transmission fluid from the converter apply circuit to the converterrelease circuit, if the torque converter is a two-pass torque converter.Alternatively, if the torque converter is a three-pass or closed-pistontorque converter, engagement of the torque converter lock-up clutch mayinclude, at 314, the CARV adjusting a flow of transmission fluid fromthe converter apply circuit to the converter open.

If engine shut-down conditions are confirmed (at 304), then at 316,exhaust flow out of the torque converter may be restricted, for example,by at least partly closing the flow restriction valve. By closing theflow restriction valve, flow may be restricted out of a converterrelease circuit (in a two-pass torque converter) or a converter opencircuit (in a three-pass or closed-piston torque converter). At 318, theduty cycle of the torque converter lock-up clutch solenoid may beadjusted to adjust the actuation state of the clutch solenoid. In oneexample, as depicted, the solenoid may be energized. In response tosolenoid energization, at 320, the clutch apply regulatory valve (CARV)may adjust the hydraulic pressure and flow of transmission fluid to thetorque converter lock-up clutch to enable an engagement of the clutchand a consequent lock-up of the torque converter. As previouslyelaborated, this may include (at 322) adjusting a flow of transmissionfluid from the converter apply circuit to the converter release circuitin a two-pass torque converter, or (at 324) adjusting a flow oftransmission fluid from the converter apply circuit to the converteropen circuit in a three-pass or closed-piston torque converter. In theabsence of pump output, herein, by restricting flow out of the hydrauliccircuit, torque capacity of the torque converter lock-up clutch may berestored. In one example, less than full torque capacity may berestored. However, the less than full restored capacity may besufficient for locking the torque converter and applying a drag torqueon the engine.

Now turning to FIG. 4, a routine 400 is described for performing anidle-stop operation in the vehicle system of FIG. 1 using a torqueconverter lock-up clutch (TCC)-based drag torque. At 402, idle-stopconditions may be confirmed. Any or all of the idle-stop conditions, asfurther described herein, may be met for an idle-stop condition to beconfirmed. For example, it may be confirmed that the engine is operating(e.g., carrying out combustion), that the battery state of charge ismore than a threshold amount (for example, 30%), that the vehiclerunning speed is within a desired range (for example, no more than 30mph), that the air conditioner did not issue a request for restartingthe engine (as may be requested if air conditioning is desired), thatthe engine temperature is within a selected temperature range (forexample, above a predetermined threshold), that a start has not beenrequested by the vehicle driver (for example, as determined by thethrottle opening degree), etc. If idle-stop conditions are notconfirmed, the routine may end. However, if any or all of the idle-stopconditions are met, then at 404, the engine may be shut-down. At 406,the torque converter lock-up clutch may be engaged to lock-up the torqueconverter and a TCC-based drag torque (or external friction torque) maybe applied through the torque converter lock-up clutch on the engine toexpedite engine spin-down. In one example, if the vehicle is moving whenthe torque converter is locked-up, the direct coupling of the engine tothe turbine shaft may allow the vehicle braking (through the wheelsand/or wheel brakes) to directly slow down the vehicle and reduce enginespeed (for example, bring down engine speed to zero). In anotherexample, if the vehicle is stationary when the torque converter islocked-up, the engaged torque converter lock-up clutch drags the enginespeed down to turbine speed (which is zero), enabling rapid enginespin-down. As previously elaborated with reference to FIG. 3, to applythe drag torque, a controller may close a flow restriction valve (FRV)to restrict flow out of the torque converter lock-up clutch. Further,the controller may modulate the hydraulic pressure generated in, andthereby the degree of engagement of, the torque converter lock-up clutchby adjusting the clutch apply regulatory valve.

The degree of engagement of the torque converter lock-up clutch (thatis, whether the clutch is fully engaged, or partially engaged) may beadjusted in response to operating conditions, including, an engine speedand/or a desired engine stopping position. In one example, when theengine speed is above a desired engine speed, the degree of engagementof the torque converter lock-up clutch may be increased to increase thedrag torque applied. In another example, when the engine speed is belowa desired engine speed, the engagement of the torque converter lock-upclutch may be decreased to decrease the drag torque applied on theengine. In one example, the desired engine speed may include a desiredspeed trajectory enabling the engine speed to be smoothly brought downto rest. Herein, based on the desired speed trajectory, the degree ofengagement of the torque converter lock-up clutch may be adjusted.Further, torque converter lock-up clutch slippage may be used, ifrequired, to enable the engine speed to follow the desired speedtrajectory. As such, the increased engagement of the torque converterlock-up clutch and the closed position of the flow restriction valve maybe maintained until engine spin-down has been completed.

Next, at 408, engine spin-down to zero (or a predetermined thresholdnear zero, such as 50 RPM) may be confirmed. If engine spin-down is notconfirmed, the routine may return to 406 to maintain application of theTCC-based drag torque until spin-down is achieved. When engine spin-downis confirmed, at 410, the flow restriction valve may be opened toun-restrict flow of transmission fluid out of the torque converter.Further, the engagement of the torque converter lock-up clutch may bereduced (for example, the torque converter lock-up clutch may bedisengaged) to thereby un-lock the torque converter.

At 412, restart conditions (as elaborated with reference to FIG. 5) maybe confirmed. If restart conditions are confirmed, then at 416, arestart operation (as elaborated with reference to FIG. 5) may beinitiated. If restart conditions are not confirmed, the open position ofthe flow restriction valve and the reduced engagement (or disengaged)state of the torque converter lock-up clutch may then be maintained atleast until a subsequent restart is commanded.

Now turning to FIG. 5, a routine 500 is described for performing arestart operation in the vehicle system of FIG. 1 using a torqueconverter lock-up clutch (TCC)-based drag torque. At 502, restartconditions may be confirmed. Any or all of the restart conditions, asfurther described herein, may be met for a restart condition to beconfirmed. For example, it may be confirmed that the engine is inidle-stop (e.g., not carrying out combustion), that the battery state ofcharge is less than a threshold amount (for example, 30%), that the airconditioner did issue a request for restarting the engine (as may berequested if air conditioning is desired), that an emission controldevice temperature is below a threshold, that a brake pedal has beenreleased, that a start has been requested by the vehicle driver (forexample, as determined by the throttle opening degree or the acceleratorpedal position), etc. If restart conditions are not confirmed, at 504, acontroller may maintain the engine in idle-stop with the flowrestriction valve in an open position and with the torque converterlock-up clutch disengaged (or with reduced engagement). However, if anyor all of the restart conditions are met, then at 506, the engine speedmay be estimated and it may be determined whether the engine speed isbelow a threshold, such as a threshold (for example, 50 RPM) below whicha started may be actuated. If the engine speed is below the threshold,then at 508, the engine may be cranked (for example, with the help of astarter) to restart the engine. Further, the flow restriction valve maybe opened to un-restrict flow through the torque converter lock-upclutch and one or more transmission clutches (for example, a forwardclutch and/or a torque converter lock-up clutch) may be engaged totransfer the torque generated by the engine to the wheels. That is, theengine may be cranked with the torque converter lock-up clutchdisengaged, the torque converter unlocked, and the flow restrictionvalve open. Then, as the engine speed rises, with the flow restrictionvalve open, the engagement of one or more transmission clutches may beincreased.

In comparison, if the engine speed is above the threshold, then at 510,a TCC-based drag torque may be applied on the engine to rapidly reducethe engine speed to the threshold from where the engine may be cranked.That is, the flow restriction valve may be closed and the engagementstate of the torque converter lock-up clutch may be modulated toincrease a drag torque on the engine to reduce the engine speed. Then,once the engine speed has dropped, the flow restriction valve may bereturned to the open state and the torque converter lock-up clutch maybe disengaged, and an engine restart and crank may ensue.

Now turning to FIG. 6, map 600 depicts example engine shutdown andrestart scenarios with a plurality of graphs 602-610.

An indication of idle-stop status (0 or 1) is provided in graph 602.Graph 604 depicts changes in engine speed during the example engineidle-stop and/or restart operations. Graph 606 depicts changes in clutchpressure (and hence, engagement status) of a torque converter lock-upclutch (TCC) during the idle-stop and subsequent restart operations.Graph 610 provides an indication of the open (O) or closed (C) status ofthe flow restriction valve (FRV).

At t₃, an idle-stop request (1) may be confirmed (for example, byconfirming idle-stop conditions) and an idle-stop operation may beinitiated. Herein, an idle-stop engine shut-down operation may beperformed in response to the presence of idle-stop conditions andwithout the vehicle operator requesting an engine shut-down (forexample, by turning off the vehicle ignition, and hence the engine). Asdepicted, the idle-stop conditions may prevail until t₅ when asubsequent restart request is confirmed. However, the majority ofadjustments required to attain an engine shut-down upon confirmation ofidle-stop conditions, may be performed between t₃ and t₄. At t₅,idle-stop may be stopped due to confirmation of restart conditions.While restart conditions may prevail long after t₅, the majority ofadjustments required to attain an engine restart upon confirmation ofrestart conditions, may be performed between t₅ and t₆.

At the time the idle-stop is requested (t₃), the engine may be runningat a higher engine speed (as shown in 604), the flow restriction valve(FRV) may be open (as shown in 610), and torque converter lock-up clutch(TCC) may be partially engaged (as shown at 608) or alternatively may bedisengaged. At t₃, when idle-stop conditions are confirmed, the enginemay be shut-down and a reduction of engine speed may be initiated withthe help of an increased TCC-based drag torque to thereby expediteengine spin-down. Specifically, in response to the idle-stop request,FRV may be closed to thereby enable the clutch capacity of the TCC to beimproved in the absence of a pump-driven hydraulic output. With FRVclosed, the TCC pressure may be increased (by example, by adjusting flowthrough a converter apply regulatory valve) to enable an engagement ofthe clutch and a lock-up of the torque converter. That is, whenidle-stop conditions are met, a controller may increase restriction ofthe flow restriction valve, and while maintaining the increasedrestriction, the controller may adjust the degree of engagement of thetorque converter lock-up clutch to thereby adjust the torque converterdrag torque applied on the engine. In one example, as depicted, themaximum clutch pressure and capacity attainable during idle-stopconditions may be less than the full capacity attainable during enginerunning conditions when a system pump is running. In the depictedexample, the engagement of TCC may be further adjusted responsive toengine speed. For example, when the engine speed is above a threshold(Np), the clutch pressure and engagement of TCC may be increased toincrease the drag torque applied and when the engine speed drops belowthe threshold, the clutch pressure and engagement of TCC may bedecreased to decrease the drag torque applied. For example, when theengine speed is below the threshold, the TCC may be disengaged. Further,when the engine speed drops below the threshold, FRV may be opened toexpedite the decrease in engagement, or disengagement, of the TCC. Inthis way, using the transmission drag torque, the speed of the enginemay be reduced to zero engine speed by t₄.

Once engine spin-down is attained, FRV may remain opened and TCC mayremain disengaged until an engine restart is requested. In response to asubsequent engine restart request, for example at t₅, the engine may becranked to restart the engine with the torque converter clutch in thedisengaged (or reduced engagement condition) and with the flowrestriction valve open. Following engine restart, FRV may remain open toallow the output of the system mechanical pump to be used for clutchpressure modulation. During the engine restart, one or more alternatetransmission clutches, such as a forward clutch may be engaged (notshown), so that an engine restart is attained by t₆ with a desiredengine speed profile. Once the restart has been established, theclutches may be maintained in the engaged state with the FRV maintainedopen.

In comparison, a TCC torque modulation may be requested during enginerunning conditions, such as for example during a gear shift, as depictedin FIG. 6 at t₁. Herein, in response to the torque modulation request,FRV may be maintained in the open state. As the clutch pressure of TCCis increased between t₁ and t₂, a small drop in engine speed may beexperienced. Following the gear shift (after t₂), the clutch pressuremay be decreased to return the clutch to a less engaged (or disengaged)state.

In this way, a torque converter lock-up clutch may be engaged duringengine idle-stop conditions to expedite engine spin-down. By restrictingflow out of the torque converter clutch during engine shut-down andpump-off conditions, the torque converter lock-up clutch capacity may berestored to thereby enable a larger and longer engagement of the clutchduring the pump-off conditions. By expediting engine spin-down, fueleconomy benefits may be achieved and the quality of frequent start/stopsmay be improved.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1-16. (canceled)
 17. A vehicle system, comprising: an engine; a torqueconverter coupled to a torque converter lock-up clutch; a flowrestriction valve configured to restrict flow of transmission fluid outof the torque converter; and a control system configured to selectivelyshut-down the engine during engine idle-stop conditions withoutreceiving a shut-down request by an operator, where the control systemcloses the flow restriction valve and increases engagement of thelock-up clutch to apply a drag torque on the engine and rapidly stop theengine for the shut-down.
 18. The system of claim 17, wherein the flowrestriction valve is positioned in a converter release circuit when thetorque converter is a two-pass torque converter, and is positioned in aclutch out circuit when the torque converter is either a three-passtorque converter or a closed-piston torque converter.
 19. The system ofclaim 17, wherein after engine shut-down, the control system is furtherconfigured to open the flow restriction valve and decrease theengagement of the lock-up clutch.
 20. The system of claim 19, whereinduring a subsequent restart, the control system is configured to restartthe engine with the flow restriction valve open and with the lock-upclutch in the decreased engagement condition.